This invention relates to the preparation of 1,3-propanediol based polyesters such as polytrimethylene terephthalate (hereafter PTT). More particularly this invention relates to a process for achieving a degree of crystallization of PTT which will prevent pellet blocking and agglomeration. In one aspect, the invention relates to a process that can be carried out in a continuous manner, as well as batch. In another aspect, the invention relates to an apparatus for the continuous crystallization of polytrimethylene terephthalate (PTT).
Polytrimethylene terephthalate is a polyester useful in fiber applications in the carpet and textile industries. The manufacture of polytrimethylene terephthalate involves the condensation polymerization of 1,3-propanediol and terephthalic acid to a polymer having an intrinsic viscosity (hereafter referred to as IV) of about 0.4 to 1.0 dl/g. The polymer melt is discharged from the melt reactor and extruded through an extrusion die into strands. The strands are quenched in cold water and cut into pellets for storage or transportation.
It has been found that polytrimethylene terephthalate pellets tend to adhere together, or block, during storage or shipping at temperatures above the polymer glass transition temperature Tg (about 45xc2x0 C.), which temperature can easily be reached during storage in a silo, rail car, or hopper. Agglomeration of the pellets can also occur during drying.
In accordance with the foregoing, the present invention is a continuous process and apparatus for crystallizing pellets of polytrimethylene terephthalate in order to prevent blocking which comprises:
a) introducing polytrimethylene terephthalate pellets having an intrinsic viscosity of at least about 0.4 dl/g into a conduit containing a liquid which is moving through the conduit, thereby causing the pellets to move through the conduit with the liquid;
b) adjusting the temperature of the pellets and the liquid to a temperature of about 50 to about 95xc2x0 C. for a time sufficient to induce a degree of crystallinity of at least about 35% in the pellets; and
c) separating the pellets from the liquid.
The process is preferably carried out in a continuous liquid pellet suspension apparatus comprising, for example, a hot water crystallization (HWC) pipe, at a sufficient flow rate to retard settling of pellets. The desired pellet properties are generally reached in a residence time in the conduit within the range of about 3 seconds to about 5 minutes. It is preferred that weight ratio of the liquid to the pellets be from about 5:1 to about 200:1, most preferably about 10:1 to about 100:1.
In one aspect of the practice of the invention, crystallized pellets are cooled to a temperature below their glass transition temperature during classification to remove fines and oversized pellets. The combined classifier-cooler includes a screen for removing pellet fines, dust and undersized pellets, a slice plate section having air flowing through, preferably from underneath, to cool the pellets, and a perforated plate through which pellets of the desired size will pass and which retains oversized pellets.